Understanding devastating earthquakes

Most earthquakes occur in the upper 20km of the Earth’s crust,
where the rocks are cold, brittle, and elastic, able to build up the
tectonic stresses released in sudden earthquake events. Below
this depth, the lower crust is hotter and rocks there typically
deform plastically and steadily, rarely accumulating enough
stress for large earthquakes.

Despite this, some of the largest
earthquakes can initiate in the lower crust,
and improving our understanding of these
deeper and potentially devastating earthquakes
is the aim of a NERC-funded University of Plymouth based
project involving Luca Menegon, Lucy
Campbell and Iain Stewart.

Image credit: Heidi Morstang

In July 2017 the project research team visited the
Lofoten Islands in Norway, where an
exceptionally well-preserved field site provides
a rare ’window’ into these once-deep rocks, now
exposed at the earth’s surface after millions of
years of erosion. We were joined by filmmakers
Heidi Morstang and Patrik Säfström, who wanted
to capture a visual record of our ancient seismic
investigations. Although this interdisciplinary
coupling was a rarity for the geology team, it was
great fun to be part of the filming process – we
simply continued working as usual, and it soon
felt very normal to have a camera in amongst the
field measurements and discussions.

As for the geology, we were able to map out the
fault zone network, assessing where and
when geologically ancient earthquakes had
occurred, how powerful they were, and how they
themselves may have modified the behaviour
of the fault zone. Samples were taken to cut up
for detailed analysis back in the lab; using high
magnification electron microscopes allows us to
investigate the microscopic rock structure and to
calculate the stresses and slip rates experienced
by the faults. Gathering information at different
scales allows us to gain insights into what factors
control lower crustal earthquake formation, and
how the presence of earthquakes can in turn
change the strength and behaviour of deep
crustal rocks. Ultimately, information derived
from these ancient fault roots will be used to infer
processes occurring at depth along faults that
are currently active in modern earthquake zones.